While the immune system is perhaps best known for its ability to fight off infections, there is increasing evidence that the immune system can participate in combating cancer as well. If the immune system can be directed to fight cancer, it may prove to be both more effective and less toxic than chemotherapy. Immune cells such as tumor associated macrophages (TAMs) can represent up to 50% of a tumor mass and have been shown to contribute to chemoresistance. Macrophages are a heterogeneous population of cells and can broadly be divided into M1 macrophages, which are a potent defense against tumor cells, and M2 macrophages, which instead tend to support tumors. TAMs are generally considered to have M2 properties and promote tumor progression, metastasis, and resistance to chemotherapy. Clinically, a high tumor density of TAMs has been significantly associated with resistance to chemotherapy and a worse clinical outcome in the majority of human and mouse tumors. The pro-tumor role of TAMs has been well characterized but an understanding of how TAMs induce chemoresistance is lacking. Therefore, we propose to study the biology and mechanisms of how TAMs contribute to tumor promotion and chemoresistance utilizing a novel assay developed in our laboratory called BH3 profiling. BH3 profiling is used to determine whether a cell is relatively close to the threshold of cell death (relatively primed for death), r relatively far from the threshold (unprimed). We hypothesize that M1 macrophages will prime tumor cells, making them closer to the threshold of cell death and that M2 macrophages will induce an unprimed tumor cell phenotype. The long-term objectives proposed in this application are focused on activating components of the immune systems to activate a long-term anti-tumor response. This is in line with the mission of The National Cancer Institute, which among other things, supports research and training with respect to the treatment of cancer, specifically by providing training grants to research projects conducted by universities and hospitals. An important part of this proposal is that using what we learn, we will investigate strategies to convert pro-tumor M2 macrophages to anti-tumor M1 macrophages. Such a strategy would undermine the support that macrophages already in the tumor give to tumor cells and instead convert it to a coordinated attack on the tumor by the immune system. The results obtained from this project will have a considerable effect on future research projects in the field of tumor immunology as this project will provide evidence that activating macrophages during chemotherapy has great therapeutic implications. The unique and novel clinical focus of harnessing TAMs for anti-cancer therapy has the potential to have a considerable impact in cancer treatment including the possible eradication of primary and metastatic cancer.